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    "### 8.1 Non-linear hypotheses\n",
    "\n",
    "* why do we study neural networks?\n",
    "* 参数过多，使用回归模型拟合非线性边界，会导致巨大的运算量\n",
    "* Too many parameters, using a regression model to fit the nonlinear boundary will result in a huge amount of calculation and too many parameters\n",
    "\n",
    "![](../pic/QQ截图20201024201412.png)\n"
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    "### 8.2 Neurons and the brain\n",
    "\n",
    "* input layer $\\Rightarrow$ hidden layer $\\Rightarrow$ output layer\n",
    "\n",
    "$a_i^{(j)} = $ layer (j), unit (i)\n",
    "\n",
    "$\\theta^{(j)} = $ weights from layer (j) to layer (j+1)\n",
    "\n",
    "\n"
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